The recovery of C1 to C4 carboxylic acids (hereinafter “lower acids”) from aqueous streams is a common industrial problem arising from a variety of reaction and processing steps. Simple distillation of wet acid streams to recover glacial acids is hampered by unfavorable vapor-liquid equilibrium (VLE) and high energy costs with all C1 to C4 carboxylic acids. Examples of unfavorable VLE include the formic acid-water maximum-boiling homogeneous azeotrope, the acetic acid-water VLE “pinch” (a region of low relative volatility), and the minimum-boiling homogeneous azeotropes with water and all C3-C4 carboxylic acids.
Various approaches have been suggested in the art to address the problem of lower acid recovery from wet acid feeds. For example, one approach subjects an aqueous lower acid solution to azeotropic distillation together with an entraining component capable of forming a heterogeneous minimum-boiling azeotrope with water, so that the azeotrope boils at a temperature substantially lower than pure water, the pure lower acid, and any acid-water azeotrope. An extraction step often precedes the azeotropic distillation. The extraction step partitions the carboxylic acid into a water-immiscible solvent (which is often the same as the azeotropic entrainer) in order to remove the bulk of the water from the recovered acid. Many examples of azeotropic distillation, extraction, and combinations thereof using conventional organic solvents have been proposed in the art. These include U.S. Pat. Nos. 1,839,894; 1,860,512; 1,861,841; 1,917,391; 2,028,800; 2,050,234; 2,063,940; 2,076,184; 2,123,348; 2,157,143; 2,184,563; 2,199,983; 2,204,616; 2,269,163; 2,275,834; 2,275,862; 2,275,867; 2,317,758; 2,333,756; 2,359,154; 2,384,374; 2,395,010; 2,537,658; 2,567,244; 2,854,385; 3,052,610; and 5,662,780, and Eaglesfield et al., “Recovery of Acetic Acid from Dilute Aqueous Solutions by Liquid-Liquid Extraction—Part 1,” The Industrial Chemist, Vol. 29, pp. 147-151 (1953).
Several solvent characteristics determine the capital and energy costs of extraction-distillation processes for the extractive recovery of lower acids from wet acid feeds. The solvent for the extraction process is immiscible with water and meets two criteria:                a) The solvent shows some selectivity between extraction of the carboxylic acid and water, i.e., the ratio of carboxylic acid to water in the extraction solvent after extraction is substantially larger than in the wet acid feed stream. This factor can be quantified as the weight ratio of water to acid in the extract stream as defined in more detail below.        b) The solvent shows sufficient affinity and capacity for the lower carboxylic acid.These characteristics are quantifiable from experimentally determined equilibrium partition coefficients as defined in more detail below.        
The equilibrium partition coefficient (also used interchangeably with the term “partition coefficient”) for component A (the lower carboxylic acid) is defined as follows:
      P    ⁡          (      A      )        =            weight      ⁢                          ⁢      percent      ⁢                          ⁢      A      ⁢                          ⁢      in      ⁢                          ⁢      solvent      ⁢                          ⁢      phase              weight      ⁢                          ⁢      percent      ⁢                          ⁢      A      ⁢                          ⁢      in      ⁢                          ⁢      aqueous      ⁢                          ⁢      phase      
The partition coefficient is a measure of the relative concentrations of the solute to be extracted in the two phases. The value of the acid partition coefficient is directly related to the amount of solvent that is required to effect a given extraction. Low values of the partition coefficient indicate high levels of solvent are required, and high values of the partition coefficient indicate low levels of solvent are required. Since the acid partition coefficient changes with acid concentration, the minimum amount of solvent required to effect a given amount of acid extraction also changes. Thus, the controlling solvent flow requirement for the extraction is dictated by the lowest value of the acid partition coefficient as the acid concentration varies from the high of the inlet wet acid feed to the low of the outlet acid concentration of the exiting raffinate stream.
The controlling acid partition coefficient may be defined as:Pcont=minimum(Praff,Pextr)where
Praff=acid partition coefficient at an acid concentration approaching that desired in the raffinate stream (i.e., at low acid concentration); and
Pextr=acid partition coefficient at an acid concentration approaching that desired in the extract stream (i.e., at high acid concentration).
The most important water-acid selectivity value is that at the extract end of the extraction cascade. It is defined as:Rextr=Wextr/Aextr where
Wextr=weight fraction of water in the extract product stream; and
Aextr=weight fraction of acid in the extract product stream.
The controlling partition coefficient, Pcont, and extract water-to-acid ratio, Rextr, may be combined to yield an overall extraction factor, ε, which is a simple measure of the efficacy of a given solvent for recovering lower acids from wet acid feeds in an extraction-distillation process. The extraction factor, ε, is defined as:ε=Pcont/Rextr=(Pcont*Aextr)/Wextr 
Generally, the higher the extraction factor, the lower the capital and energy costs are for a given extraction.
Extraction solvents that exhibit the inverse behavior are also known. That is, their acid partition coefficient is lowest at the extract end of the cascade (high acid concentration) and highest at the raffinate end (low acid concentration). Examples of such solvents include nitriles, phosphate esters, phosphine oxides (U.S. Pat. Nos. 3,816,524 and 4,909,939), and amines (e.g., King, “Amine-Based Systems for Carboxylic Acid Recovery: Tertiary Amines and the Proper Choice of Diluent Allow Extraction and Recovery from Water,” CHEMTECH, Vol. 5, pp. 285-291 (1992); and Tamada et al., “Extraction of Carboxylic Acids with Amine Extractants. 2. Chemical Interactions and Interpretation of Data,” Ind. Eng. Chem. Res., Vol. 29, pp. 1327-1333 (1990)).
This inverse behavior (partition coefficient highest at low acid concentration) has also been observed for a phosphonium- and an ammonium-phosphinate ionic liquid (Blauser et al., “Extraction of butyric acid with a solvent containing ammonium ionic liquid,” Sep. Purif. Technol., Vol. 119, pp. 102-111 (2013); Martak et al., “Phosphonium ionic liquids as new, reactive extractants of lactic acid,” Chem. Papers, Vol. 60, pp. 395-98 (2006)) and a phosphonium carboxylate salt (Oliveira et al., “Extraction of L-Lactic, L-Malic, and Succinic Acids Using Phosphonium-Based Ionic Liquids,” Sep. Purif. Tech., Vol. 85, pp. 137-146 (2012)).
In 2004, Martak and Schlosser introduced using phosphonium phosphinate ionic liquids for extracting certain carboxylic acids from aqueous solutions with abstracts at the May 24-28 Meeting of the Slovakian Society of Chemical Engineering (SSCHE) in Tatranské, Slovakia. Jan Martak and Stefan Schlosser, “Screening of ionic liquids for application in solvent extraction and pertraction,” 31st Int'l Conf. of SSCHE, p. 188 (2004). In this abstract, the authors reported that butyric acid and lactic acid could be extracted by a material referred to as “IL-A”, or IL-A in dodecane, much more effectively than in other ionic liquids, such as ethylmethylimidazolium bis(trifluoromethyl)amide (emim-NTf2), octylmethylimidazolium hexafluorphosphate (omim-PF6), and others.
The authors published more complete data for extracting lactic acid with IL-A later that year in the XIX ARS Separatoria (from Zlotky, Poland). Jan Martak and Stefan Schlosser, “Ionic Liquids of Pertraction and Extraction of Organic Acids,” 19th ARS Separatoria, pp. 106-113 (2004). Two subsequent papers by Martak and Schlosser on the extraction of lactic acid by a phosphonium ionic liquid are consistent with the identity of IL-A being what is known as IL-104 (trihexyl(tetradecyl)phosphonium bis(trimethylpentyl)phosphinate) from Cytec Industries. Jan Martak and Stefan Schlosser, “Phosphonium Ionic Liquids as New, Reactive Extractants of Lactic Acid,” Chem. Papers, Vol. 60, pp. 395-98 (2006); Jan Martak and Stefan Schlosser, “Extraction of Lactic Acid by Phosphonium Ionic Liquids,” Sep. Purif. Technol., Vol. 57, pp. 483-94 (2007). The structure of IL-104 (or P666,14-phosphinate) is shown below.

In the 2006 paper, the authors graphically presented an interesting characteristic of lactic acid extraction by IL-104. It had much stronger partitioning of lactic acid at lower concentrations (see FIG. 11). We describe this as “inverse” because it is more common for covalent organic solvents to have lower partitioning of carboxylic acids at lower aqueous concentrations.
The work of Luis Rebelo and coworkers, however, brought this detail for lactic acid extraction by IL-104 into question. Rebelo et al. reported the performance of three different phosphonium ionic liquids: (1) IL-104; (2) IL-101 (in which a chloride replaced the phosphinate anion); and (3) a third ionic liquid (in which a decanoate anion replaced the phosphinate anion). F. S. Oliveira et al., “Extraction of L-Lactic, L-Malic and Succinic Acids Using Phosphonium-Based Ionic Liquids,” Sep. Purif. Technol., Vol. 85, pp. 137-46 (2012). Rebelo et al. showed in FIGS. 2 and 3 that the partitioning of lactic acid between IL-104 and water was diminished at lower (aq) lactic acid concentrations. Thus, some degree of uncertainty was cast on the interesting and opposing observations of Martak and Schlosser. Rebelo et al. did have difficulty with phase behavior and mass accountability of lactic acid, which they attributed to coordination of lactic acid to the phosphinate anion. Formation of a third phase and inverse miscelles also complicated a simple understanding of the studies by Rebelo et al.
In 2008, Martak and Schlosser reported the extraction of butyric acid with IL-104 in dodecane. Jan Martak and Stefan Schlosser, “Liquid-liquid equilibria of butyric acid for solvents containing a phosphonium ionic liquid,” Chem. Papers, Vol. 62, pp. 42-50 (2008). That system also displayed the inverse behavior described above with higher partitioning at lower concentrations (see FIG. 2). In 2013, the authors substituted an ammonium cation (trioctylmethylammonium) for the phosphonium of IL-104, as shown below. M. Blahusiak et al., “Extraction of butyric acid with a solvent containing ammonium ionic liquid,” Sep. Purif. Technol., Vol. 119, pp. 102-11 (2013).

Although the ammonium phosphinate had much greater mutual solubility of water (>20%), much lower thermal stability (decomposing significantly above 150° C. versus 250° C. for the phosphonium phosphinate), and lower solubility in dodecane; it did display the same attractive feature of having a higher extraction efficiency at lower butyric acid concentrations.
Despite the work of Martek et al. and Rebelo et al., there continues to be a need in the art for extraction solvents with excellent partitioning of lower carboxylic acids from aqueous solutions and that enable the simple separation of these acids. There is also a need for extraction solvents with high extraction factors whereby C1 to C4 carboxylic acids can be recovered from wet acid feeds in an energy-efficient and cost-effective manner.
The present invention addresses these needs as well as others, which will become apparent from the following description and the appended claims.